[ASan] Make GlobalsMD member a const reference.

PR42924 points out that copying the GlobalsMetadata type during
construction of AddressSanitizer can result in exteremely lengthened
build times for translation units that have many globals. This can be addressed
by just making the GlobalsMD member in AddressSanitizer a reference to
avoid the copy. The GlobalsMetadata type is already passed to the
constructor as a reference anyway.

Differential Revision: https://reviews.llvm.org/D68287

llvm-svn: 373389